The adsorption of tetraalkylammonium (TAA) ions on silica shifts the isoelectric point to higher pH values. The IEP shift increases with increasing alkyl chain length. Also, tetraethylammonium (TEA) and tetrapropylammonium (TPA) ions can be adsorbed in larger amounts than tetramethylammonium (TMA) ions. The chemisorption of TAA ions can be explained by assuming an attraction between hydrophobically hydrated regions around surface siloxane bridges and around the TAA ions. The adsorption data of TMA ions are compatible with the stimulated adsorption model.